1. Field of the Invention
The present invention relates to a method for producing platinum nanoparticles with nanometer sizes and polyhedral shapes.
2. Description of Related Art
Recently, it has been known that platinum nanoparticles with polyhedral shapes are excellent in performance such as catalytic activity, etc., as compared with spherical platinum nanoparticles. “Chemistry of Materials, 8 (1966) (pages 1161-1163)” and “Electrochimica Acta, 52(2006) (pages 1632-1638) disclose techniques of synthesizing platinum nonoparticles through the liquid-phase reduction method using platinum complexes, and with these techniques, polyhedral platinum nanoparticles of which (100) faces exhibit high reaction activity are synthesized by selectively adsorbing sodium polyacrylate as a capping reagent on the (100) faces of platinum crystal nuclei to stop the growth of the (100) faces while promoting the growth of (111) faces. These documents disclose that carbonyl groups in side chains of sodium polyacrylate are bonded to surfaces of platinum to operate as the capping reagent.
“FUEL CELL, 5(2006)” (pages 69-72) discloses the technique of producing polyhedral platinum particles having (100) faces that exhibit high reaction activity by adding sodium iodide (NaI) along with sodium polyacrylate.
With the techniques of the above-described documents, sodium polyacrylate has been used as the capping reagent exhibiting aggregation inhibition effect. However, sodium polyacrylate exhibits a high viscosity so that it is difficult to readily wash off. Therefore, there is a limit in enhancing the recovery rate of platinum particles with polyhedral shapes. In addition, since a considerable amount of sodium polyacrylate remains in assemblies of platinum nanoparticles, the inherent performance of platinum nanoparticles, such as catalytic activity, etc. may be deteriorated.
And Japanese patent application laid-open No. 2002-42825 discloses a method for forming platinum nanoparticles on carbon black, which includes the steps of dissolving potassium tetrachloroplatinate and sodium polyacrylate in water to prepare an aqueous solution thereof, bubbling hydrogen in the prepared aqueous solution to form a colloidal solution of platinum, adding carbon black to the colloidal solution of platinum, adsorbing colloidal particles on carbon black particles after adjusting the pH of the colloidal solution of platinum to 3 through 12, and separating carbon black on which the colloidal particles have been adsorbed from the liquid. By carrying out these steps sequentially, platinum particles with cubic shapes or tetrahedral shapes are supported on surfaces of carbon black particles.
Japanese patent application laid-open No. 2007-131926 discloses a method for forming platinum nanoparticles, which includes the steps of mixing a solution of a platinum compound, specific adsorption chemical species composed of cations or anions, and sodium polyacrylate with each other, and growing platinum crystals with the deposition from the solution of the platinum compound. In embodiments of this document, an aqueous solution of sodium polyacrylate is added to a solution of platinic acid chloride, and an aqueous solution of sodium iodide or sodium sulfate is further added thereto to prepare a mixture liquid, and then, hydrogen gas is blown in the prepared mixture liquid to reduce platinic acid chloride. In this method, carbon black is mixed in a colloidal solution of platinum in which platinum nanoparticles have been formed by bubbling hydrogen. Since carbon black is mixed in the colloidal solution of platinum after the formation of platinum nanoparticles, platinum nanoparticles may not been favorably supported on particles of carbon black so as to be separated therefrom. In this case, the platinum nanoparticles may aggregate to increase the particle diameter thereof, and exposed crystal faces may vary.